Vertical MOSFETs using trenched gates are popular as high voltage, high power transistors due to their relatively thick, low dopant concentration drift layer which enables a high breakdown voltage in the off state. Typically, the MOSFET includes a highly doped n-type substrate, a thick low dopant concentration n-type drift region, a p-type body formed in the drift region, an n-type source at the top of the body, and a vertical (trenched) gate separated from the channel region by a thin gate oxide. A source electrode is formed on the top surface, and a drain electrode is formed on the bottom surface of the substrate. When the gate is sufficiently positive with respect to the source, a vertical region in the p-type body between the n-type source and the n-type drift region inverts to create a conductive path, or channel, between the source and drain.
In the MOSFET's off-state, when the gate is shorted to the source or at a negative bias, the drift region depletes, and high breakdown voltages, such as exceeding 600 volts, can be sustained between the source and drain. However, due to the required low doping of the thick drift region, the on-resistance suffers. Increasing the doping of the drift region reduces the on-resistance but lowers the breakdown voltage.
Such conventional vertical MOSFETs use trenches with substantially parallel opposing sides, where a thin gate oxide is grown on the trench walls. The oxide has a substantially equal thickness along the walls. The trench is then filled with a doped polysilicon to form the gate. The filled trenches may also be used as field plates to provide more uniform electric field distribution.
The paper by Kenya Kobayashi et al., entitled “100V Class Multiple Stepped Oxide Field Plate Trench MOSFET (MSO-FP-MOSFET) Aimed to Ultimate Structure Realization” (Proceedings of the 27th International Symposium on Power Semiconductor Devices & IC's, pp. 141-144), describes trenches with a variable thickness oxide layer, where the oxide is thicker towards the bottom of the trench. The trenches are then filled with doped polysilicon. FIG. 1 is reproduced from this paper and shows a vertical MOSFET having an n+ drain 12 (which may be the substrate), an n-drift region 14, generally rectangular trenches 16, tapered oxide 18 lining the trenches 16, doped polysilicon forming gates 20, a p-body 22, an n+ source 24 over the p-body 22, a top source metal layer 25 connected to the n+ source 24 and p-body 22, and an oxide 26 insulating the gates 20 from the source metal layer 25. A gate metal electrode (not shown) is connected to the gates 20. In a typical operation, a positive voltage is applied to the drain 12, and one terminal of a load is connected to the source metal layer 25. Another terminal of the load is connected to ground. When the gate 20 is biased above the threshold level, the p-body 22 inverts to conduct a current vertically between the source 24 and the drain 12. When the gate 20 is shorted to the source metal layer 25, the thick drift region 14 supports the electric field. The relative low doping of the drift region 14 is required for a good breakdown voltage but increases on-resistance. When the gates 20 are shorted to the source metal layer 25, they act as field plates, as described below.
By providing a thicker oxide 18 near the bottom of the trenches 16, where the electric field is highest when the MOSFET is off, the oxide insulation can withstand a higher voltage field, compared to a conventional thin gate oxide. The oxide is thin near the top of the trench 16 next to the channel region (p-body). The grounded gate 20 acts like a field plate to uniformly distribute the electric field in the drift region 14 by laterally depleting the drift region 14, which increases the breakdown voltage. In other words, the depletion region in the drift region 14 (in the mesa) between the trenches 16 is more uniform.
The Kobayashi paper describes how the oxide in the trench 16 is tapered by successively growing new oxide layers in the trench and etching each new oxide layer back to a different depth, so the older oxide layers remaining after each etch add to the overall thickness of the oxide layer at the different depths. This process is very time-consuming and can realistically only be used to form an oxide layer having only a few stepped thicknesses.
New techniques are needed for forming tapered oxides in a trench that do not have the drawbacks of the technique described in the Kobayashi paper.